The introduction of fuel into the cylinders of an internal combustion engine is most commonly achieved using fuel injectors. A commonly used injector is a closed-nozzle injector which includes a nozzle assembly having a spring-biased needle valve element positioned adjacent the injector nozzle for allowing fuel to be injected into the cylinder of an internal combustion engine. The needle valve element moves to allow fuel to pass through the injector nozzle and out the injector orifices or spray holes, thus marking the beginning of the fuel injection event. Fuel injector designs may include reduced nozzle orifice diameters and increased injection pressures to provide increased engine power density and reduced emissions. However, over time, the power resulting from such modern injector orifice and/or spray-hole geometries may degrade as a result of formations of carbonaceous deposits (also called carbon build up or coking) on fuel injector nozzles.
Increased fuel injector nozzle deposit formation occurs when the temperature at the nozzle tip rises. As is known in the art, when zinc levels reach a critical concentration in the fuel, significant coking occurs within a relatively short period. Internal and external deposits on fuel injector nozzles can negatively impact engine behavior as well as produce increased acoustic and pollutant emissions in diesel engines with direct injection. A variety of studies involving direct injection diesel engines show a deterioration of combustion and mixture formation as a direct result of carbonaceous deposits accumulating on the injector nozzle. Moreover, the deposits can also cause an increase in fuel consumption and a reduction in the power output of direct injection diesel engines.
Prior art attempts to mitigate injector nozzle coking have included running the engine with zinc-free fuel to partially reverse the nozzle coking deposition. Chemical mechanisms such as oxidation may also be employed to destroy the organic compounds present in the carbonaceous particles. Moreover, evaporation and desorption may be utilized to reduce the gaseous fraction dissolved in the deposits, however in both approaches an additional step of abrasion is necessary to cause the required breaking-off force to facilitate removal of the carbonaceous deposit layer. Detergent additives to the diesel fuel may also be effective at removing carbon deposits from fuel injector nozzles; however these additives can be expensive and difficult to dispense accurately.